The most fundamental building block of the classical computer is the
bit. A bit is a variable with only two possible values: 0 or 1. The
smallest conceivable storage for a bit involves a single elementary
particle of some sort. Consider a particle with a spin-1/2
characteristic that when measured is either +1/2 or -1/2. We
could encode 1 to be +1/2 and 0 to be -1/2, and if we could
measure and manipulate the spin of such a particle, then we could
theoretically use this particle to store one bit of information. The
spin-1/2 particle, or any two state system that behaves in a quantum
manner, could instead be the fundamental building block of a quantum
computer. We will call a two state quantum system a qubit, to
denote that it is analogous to a classical bit. When a classical bit
is measured, the value observed will always be the value stored.
Quantum physics states that when we measure a qubit we will find it in
one of two states, which we can label 0 and 1. The differences
between the qubit and the bit arise from the possible state of a qubit
between measurements.